Without limiting the scope of the invention, its background is described in connection with current anisotropic etch techniques.
The fabrication of modern integrated circuits often requires the patterning of materials to very small dimensions and very stringent tolerances. At some stage of virtually any process of manufacturing integrated circuits or discrete devices, many materials including crystalline and non-crystalline semiconductors, insulators and metals must be formed in very precise patterns. A common technique is to deposit a continuous film of the required material, then form an etch mask on the surface of the material (commonly a photoresist layer which has been patterned with a photolithographic technique), and then to etch away the portions of the film not covered by the etch mask, leaving the desired pattern.
Clearly, an etch technique should not etch away material under the etch mask if the pattern defined by the mask is to be preserved in the underlying material. Any etch for this purpose should therefore be largely directional (anisotropic). The desired etch direction is usually normal to the surface of the substrate (i.e. the etch proceeds down into the underlying material but not laterally underneath the mask). An anisotropic etch will ideally leave a virtually vertical sidewall under the etch mask edge.
Plasma etches are among the dry etches which are widely used and are made anisotropic largely by the direction of the applied electric field. Liquid phase chemical etches (wet etches) are generally assumed to be nondirectional (isotropic), and therefore have not found widespread use in the manufacture of VLSI devices. While wet etches are desirable because they are generally low energy techniques which cause little or no damage to substrate materials, unfortunately only a few anisotropic wet etches are known. Crystalline silicon, for example, may be anisotropically etched to some degree with potassium hydroxide. Some chlorine reagents will directionally etch gallium arsenide. These orientation dependent etch (hereafter referred to as ODE) methods are anisotropic because the etch rate along one crystal orientation is faster than that of other directions.